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What is the "correct" way of calculating amps on multiple 12V rails?

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I've been trying to figure this out for a while now, because it seems like everyone has a different formula for determining the total number of amps on a multiple 12V rail PSU. I think I may have found the "correct" way to do it, but I would like some opinions/confirmations on this method.

My PSU is an Antec Earthwatts 430W (12V1 = 16A, 12V2 = 17A).

Some people say in order to find the combined total amperage between the two rails, you simply add them together (16A + 17A = 33A). I don't think that's true, based on maybe a 70-30 ratio of forum repliers saying the opposite is true.

Some people say you take the total number of watts and divide it by 12V (430W/12V = 35.8A). Again, maybe a 70-30 ratio of people seem to disagree.

Still others say you take the total number of watts multiplied by the PSU's efficiency divided by 12V [(430W x 0.82)/12V = 29.4A]. I don't think this is right, given I've only seen this theory once.

Some people say you take the total number of watts that the 12V rail(s) can use (384W according to the side of my PSU case) and divide it by 12V (384/12V = 32A). But this is the number of amps for all components in the system utilizing the 12V rail, so the actual number of amps available to, say, a new GPU, would be (384W - wattage of 12V components = X, and X/12V = Amps available to the new GPU. This is the method I personally think is right, although I've seen it the least often.

Finally, to throw a wrench in the whole thing, I've seen several statements on forums and youtube saying that there is a "cap" for any one individual rail, in which, if you have multiple 12V rails, each individual rail may only use 20A maximum, and that one of those rails must solely provide power to the CPU only (that last part varies from source to source). Therefore it would be best to get a PSU with a large single 12V rail so that more amps can be used for a high-end GPU. I just can't see this one being true, but maybe I misunderstood.

So my question is.............what the frick is it? For pity sake, why should this be that hard to figure out? I mean I've researched it guys, and this is the best a PC newb like me can come up with. No wonder 90% of power supplies get horrible ratings on Newegg, everybody's fryin' their computers cuz no one knows what the numbers mean! Some people "insist" you use a 9500GT with a 600W PSU, while Paul from the Newegg youtube channel had this 430W PSU chuggin' away with an i5 2500k and a GTX 550ti GPU at full load at less than 160W. I mean come ON!

Ugh, any help would be greatly appreciated, thank you for reading this long post/rant!
a b ) Power supply

The answer is right in front of you.
"My PSU is an Antec Earthwatts 430W (12V1 = 16A, 12V2 = 17A). "

You have one rail capable of 16a*12v=192w and the other capable of 204w. They act just like seperate power supplies. Unless you have something telling you that the rails are actually combined, like Johnnieguru sometimes finds, then the max you can run is 17amps from the one rail (like if you have a video card that needed 25A then you would need a connector from each rail since you dont have a single rail that can do the job alone) The hard part is figuring out which rail is which. Often in two rail PSU's one is for the mobo connectors (24 pin and cpu power) leaving the 2nd rail to do everything else. (nice of them to do that to you huh?)

Dividing the total watts by 12 is rediculas since other power it creates is part of that total power. Like 20A on the 5v rail would be 100W - where do they think that comes from? Magic?

Adding them both together and multiplying by 12 gives you and idea of the total power (33*12= 396w) and not much more.

The PSU efficiency is factored in creating the rated power so a 430w supply at 80% would draw 537.5 from the wall at full usage. Its importaant to not that if you are not using the full 430w the it wont be pulling the 537w from the wall. If you use 100w then it might be pulling 125w from the wall if it can maintain the 80% effiency that low.

Finally You have it right. Good for you, shows you have a good brain.
"Some people say you take the total number of watts that the 12V rail(s) can use (384W according to the side of my PSU case) and divide it by 12V (384/12V = 32A). But this is the number of amps for all components in the system utilizing the 12V rail, so the actual number of amps available to, say, a new GPU, would be (384W - wattage of 12V components = X, and X/12V = Amps available to the new GPU. This is the method I personally think is right, although I've seen it the least often. "

Just bear in mind that this goes for quality PSU's. Cheap junk can practically make up numbers and they wont care or stand by them. Thats why you see us constantly referring people to the better rated brands/models that have gone thru examination and testing like on johnnieguru site.
a c 241 ) Power supply

The second to last theory is correct. The total wattage available for the 12V rail is the rated capacity of the 12V source in watts, dividing that by 12V tells you how much current is available from the 12V rails. The rails are just over current protection circuits, they clamp down when you try to draw too much current from a single rail but dont affect anything else. The Antec Earthwatts units are weird in that summing up the capacity of each rail brings you very close to the total wattage of the unit.

A nice counter example to it is the Antec HCP 1200, it has 8 12V rails each rated for 30A each(nice high OCP threshold makes it almost impossible to overload a single rail accidentally) if you were to add those together it would theoretically have 240A(2880W) between its 12V rails, now this is an absolutely outrageous number and obviously not correct, but if you look through its pictures on newegg you will find the picture of the load table, below the 8 12V rails it says the combined wattage is 1188W(99A) this is a much more reasonable number.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

If the unit says what the combined 12V current or wattage is then it can be accurately determined, if it doesnt then there is no way to accurately determine it, for low powered units i usually sum them up and multiply by 80% if they are within a reasonable range. Any good unit will list the combined power of the rails.

The final theory has some validity except that its based on older ATX specs, older versions stated a max of 240VA per rail, or 20A max per 12V rail, that restriction has been lifted as has the rule that the CPU must be on its own rail so most PSUs you see these days do not obey that rule. Also, PSU manufacturers already split the cables between rails appropriately so that if you arent using adapters you will have a hard time overloading a rail, they spread the PCI-E connectors between rails so in general a GPU will actually be drawing power from multiple 12V rails in a multirail PSU.

Edit: From the Jonnyguru PSU FAQ
Multiple +12V rails, as it pertains to computer power supplies, typically doesn't mean there are multiple +12V rails (although a few high powered units actually may have two individual +12V outputs). Computer power supplies typically take their single +12V output and split it up into two, three, four or more +12V circuits. Each of these circuit have an "OCP", which stands for "Over Current Protection" on them that limits how much power each rail can deliver. Even on the rare occasion of a power supply with true multiple +12V outputs, it's not uncommon for these outputs to be split up two to three more times producing five or six +12V rail
http://www.jonnyguru.com/modules.php?name=NDFAQs&op=FAQ...

The units with multiple 12V sources they refer to are units like the Corsair HX 1000 which was literally 2 500W units shoved into the same box.
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a b ) Power supply

When you have multiple +12 V rails. Alittle complicated but follow:
Ex PSU with following ratings:
PSU 550 W total
+12V: 500 Watts (+12V1 18A max, +12V2 16 Amps Max, and +12V3 12Amps max
+5V and +3.3 V 100 Watts.

(1) the Amps per rail Add toward total. The total available current should be 18+16+12 = 46 Amps (Power therefore = 46 Amps x 12 V= 552 Amps. NOW for the constrains:
(A) this does not mean that if you only use 16Amps on V1 that V2 can "Borrow" the extra and Have 18A.
(B) if the +12 V has a max wattage of say 500 AMPs, then you cannot draw the max/rail. Ex. if +12V1 draws 17 Amps and +12V2 draws 15 Amps, then +12V3 can only draw 9.67Amps (even though it's listed @ 12Amps).
To throw another monkey wrench in - is that that is assuming the +12 V is +12.00. (If +12 V is really 12.4 V then 500 Watts is 40.32 Amps, not 41.67 Amps which means that +12V3 is now limited to an even lower value, 8.3A.

(C) the Next constraint that may even lower further. Lets say that the +5V and the +3.3 V have a power consumption of 75 W (less than the advertized 100 watts.
Remember total for the PSU was 550 so 550 -75 = 475 Watt. So now we have knocked that +12 V total from 500 Watts (listed) to a 475 Watts (Real available). And now that +12V3 is down to only having 7 1/2 Amps avail (ALyhough listed @ 12 Amps.

Hope my simple explanation helped, Face it mud can not get much blacker.

Wow While I was slowly typing I see 2 others beat me to the punch.

One point On the 20 Amps per rail spec. It's still there, just some PSUs ignore. However the load distribution wil limit the current per Line (really pair + wire and return wire are in series) to around 10 Amps. The reason is a limitation based on the max IsqR drop across the male/female power pins on the various MB connectors (ie the 4/8 pin ATX and the 20/24 Pin connectors and the pci-e connectors on the GPU units.

a c 117 ) Power supply

The simple way is to look at the label on the psu. Modern psu's will have a total wattage of the combined rails.
In the case of the Antec earthwatts 430, it is 384watts. Divide this by 12v, and you get 32a. It will usually be a bit less than the sum of the individual rails(16a +17a)
http://www.newegg.com/Product/ImageGallery.aspx?CurImag...

Older units with incomplete numbers may need some guesswork.
a c 77 ) Power supply

Quote:
Some people say you take the total number of watts that the 12V rail(s) can use (384W according to the side of my PSU case) and divide it by 12V (384/12V = 32A). But this is the number of amps for all components in the system utilizing the 12V rail, so the actual number of amps available to, say, a new GPU, would be (384W - wattage of 12V components = X, and X/12V = Amps available to the new GPU. This is the method I personally think is right, although I've seen it the least often.

Of course it would be the number for all components using the 12V rail, they have to get power from somewhere right? When you subtract you need to subtract the wattage of the other devices. 384 - CPU TDP = 2xx. 2xx - ~50W for mobo and system = 210. 210W / 12V = 17.5A for the GPU.

Quote:
I've seen several statements on forums and youtube saying that there is a "cap" for any one individual rail, in which, if you have multiple 12V rails, each individual rail may only use 20A maximum, and that one of those rails must solely provide power to the CPU only (that last part varies from source to source). Therefore it would be best to get a PSU with a large single 12V rail so that more amps can be used for a high-end GPU.

There is the theory of something called "trapped power". It works like this. If 12v1 powers your CPU/board and it gets 16A in the case of your powersupply, then that 16A is ONLY for 12v1 and you'll never get it back. If you have a 95W CPU, and another 35W powering your board, then you have 130W on your 12v1. 130W / 12V = 10.8A. The trapped power theory states that because this 16A is "hardwired" to the 12v rail it can't be used anywhere else, and that other 5.2A is lost. This is ofcourse wrong, as there is only a single 12V transformer in nearly all current PSUs. The rails are created by using Over Current Protection (OCP) which senses when X amount of Amperage is traveling down the line and then will stop the rail from working. A single rail PSU has no OCP, and in the event of a short will allow all amps to enter X device. If anything a multi rail PSU provides the same power as a single rail, with protection against shorts.

The reason why this happens is because people don't know what they are talking about. If you really have researched this you should have found a case where someone says "260W load * 80% efficiency = (260 * .8) 208W from the PSU.) While I disagree with some of what popatim wrote, he was at least able to correctly find/use the efficiency. Just because you can turn a screwdriver and assemble a PC doesn't mean you understand power, or how L2 cache relates to FSB speeds/having an IMC, or how memory speeds effect the computer overall.

First of all let me say THANK YOU all for your very informative replies, it's exactly what I needed.

@popatim...

Quote:
You have one rail capable of 16a*12v=192w and the other capable of 204w. They act just like seperate power supplies. Unless you have something telling you that the rails are actually combined, like Johnnieguru sometimes finds, then the max you can run is 17amps from the one rail

Finally You have it right. Good for you, shows you have a good brain.

That seems to be the opposite of what 4745454b, hunter315, and RetiredChief are saying, and i'm inclined to agree with them at the moment, but you seemed like you knew what you were talking about. Maybe i just misunderstood. And yes, i take much pride in the quality of my cerebrum  .

@hunter315...

Quote:
The final theory has some validity except that its based on older ATX specs, older versions stated a max of 240VA per rail, or 20A max per 12V rail, that restriction has been lifted as has the rule that the CPU must be on its own rail so most PSUs you see these days do not obey that rule.

That clears a lot up on that aspect, i thought it seemed a bit too different from the other theories to be right haha.

@RetiredChief...

Quote:
To throw another monkey wrench in - is that that is assuming the +12 V is +12.00. (If +12 V is really 12.4 V then 500 Watts is 40.32 Amps, not 41.67 Amps which means that +12V3 is now limited to an even lower value, 8.3A.

(C) the Next constraint that may even lower further. Lets say that the +5V and the +3.3 V have a power consumption of 75 W (less than the advertized 100 watts.
Remember total for the PSU was 550 so 550 -75 = 475 Watt. So now we have knocked that +12 V total from 500 Watts (listed) to a 475 Watts (Real available). And now that +12V3 is down to only having 7 1/2 Amps avail (ALyhough listed @ 12 Amps.

Those were two excellent points you made, not too complex, but i overlooked them nonetheless. And actually, believe it or not, your post made the most sense to me while i was reading it haha.

Thank you all again for answering my question. It was bugging me because I knew there had to be some sort of "standard" for 12V rails, but the formulas differed so much from person to person that I could never figure it out. I guessed that if anyone knew for sure how it truly worked, it would be here. I'll try to pay it forward and pass along this information when I can, I know it sure helped me a lot. Thanks!
a c 77 ) Power supply

There is very little that draws power from the 5V and 3.3v rails. Harddrives currently draw from the 5v, and soon will draw from 3.3v rail. I don't know of anything that draws from the 3.3v rail. (RAM and CPU used to, but since the days of the P4 they get their power from the 12V rail.) Here is an interesting link on this.

http://www.playtool.com/pages/psurailhistory/rails.html

As for popatim I already said I didn't agree with what he wrote. He's got the basics down, but if I'm understanding him right you already pointed out something that's off.
a b ) Power supply

Was thinking that memory used the 3.3, but apperently it also is now derived from the +12V.

On HDD, I know that all 2.5 in drives only use the +5v, but I think all the 3.5 in drives still use the +12V as they are close to 10W on average for write cycle and larger platters. Current requiremt would double if they only used the 5V source.
a c 77 ) Power supply

Quote:
On HDD, I know that all 2.5 in drives only use the +5v, but I think all the 3.5 in drives still use the +12V as they are close to 10W on average for write cycle and larger platters. Current requiremt would double if they only used the 5V source.

They draw from both. The circuit board still uses the 5v rail. The motor used to spin the disks however draws from the 12V rail. I think the only other thing that draws power from the 5v rail is USB devices. (and PCI cards. I have no clue if PCIe cards use the 5v rail or not.) Someone awhile ago posted a link talking about new hdds coming that will use the 3.3v rail instead of the 5v rail for better power figures. If you use one of these you CAN'T use a molex to SATA adapter as the molex plugs are 12 and 5 volt only, no 3.3V.

hi, excuse my ignorance but I have not figured out how to calculate the total amps on two rails ... example, my power supply S-tech 600W :

Imput voltage : 180 264Vac

Frequency : 47Hz-63Hz

Current . 4A (Max.)

Inrush Current : 80A Max At cold start at 25 deg

Efficiency : 75% min. at full load

Voltage : +3.3V +5V +12V1 +12V2 -12V +5Vsb

Max load : 30A, 28A, 16A, 18A, 0.5A, 2.0A

Ripple : 50mV, 50mV, 60mV, 60mV, 80mV, 50mv

Protection : over current protection over voltage protection

how many ampere total i have on the 12V? is enough to power a GTX 560 (the new, not ti) ?

Thanks and sorry my eng...bb
a b ) Power supply

3.3 V x 30 Amps = 99 W
5 x 28 = 140 W
12 x 16 = 192 W (12V1
12 x 18 = 216 W (12V2)
-12 x 0.5 = 6 W
5 x 2 = 10 Watts

PSU is limited to 600 Watts, But you will not max out the +5V or the +3.3 V
So Max power on the +12 V is 34 amps ( 408 Watts)
The +12 V powers your ram, Cpu and GPU. These three account for the majority.
HHDs and DVD drives also use power from the =12 V (Approx 10 W per device.)

System power + 325 W, so you should be good to go, but then you could have found out by googling your GOU and reading:
http://www.guru3d.com/article/geforce-gtx-560-ti-review...
a c 77 ) Power supply

Retired, where do you get the 16 and 18A figures from? Just because the PSU claims it doesn't mean its true.
a b ) Power supply

You mean they do NOT label the PSUs correctly, gee that could fall under false advertizing (LOL).
anyway
Can only go by what is stated. Thoes are his numbers.

But yes, I know where you are coming from. If you add the wattages up you come up with approx 664 W, BUT it is only rated at 600 W. With newer systems, your not going to max out the 3.3 and 5 volt rails, so Depending on psu QUALITY the max rated for the +12 V rails should be available - No guarantee, and as I said QUALITY is very important with PSUs. Never heard of a S-tech 600W , so can not say, the OP should google the PSU and see how it stacks up.

Should have said - If the PSU is a "good" PSU, yes you should be fine.
a c 77 ) Power supply

My yahoo search returned limited results. Usually in a more eastern country of Europe. I did find an ATX vers 2.2 model, but its clearly not his as that model was listed as 20A on both rails. (was also 80 certified, his isn't if its only 75%)

In Mallolo's case I think I'll just quote Geofelts earlier post.

Quote:
The simple way is to look at the label on the psu. Modern psu's will have a total wattage of the combined rails.
In the case of the Antec earthwatts 430, it is 384watts. Divide this by 12v, and you get 32a. It will usually be a bit less than the sum of the individual rails(16a +17a)
http://www.newegg.com/Product/Imag [...] r%20Supply

Older units with incomplete numbers may need some guesswork.

thanks guys, I think that I keep with my power supply ... at most it will change later.
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